The present invention relates to the methods of protecting electrical networks for aircraft, in particular for a bipolar power distribution network, in the case where a power source supplying power to this bipolar network is isolated, in other words with said source not being connected to an electrical ground of the aircraft.
The present invention also relates to devices for protecting electrical networks for aircraft, in particular for a bipolar power distribution network, in the case where a power source supplying power to this bipolar network is isolated, in other words, with this source not being connected to an electrical ground of the aircraft, and to aircraft comprising such protection devices.
It is known that aircraft comprise a general electric network, which connects at least one electric power source to electrical loads formed by on-board systems. For that purpose, the general network is provided with a power supply network connected to the power sources, and with a power distribution network connected to the loads.
It is also known that aircraft comprise numerous on-board systems, which consume electric power, at an ever-increasing rate.
To generate electric power, aircraft conventionally comprise alternative current electrical rotating machines configured to generate electric power (and therefore operating in a generator mode) with a high voltage alternating current (HVAC) and/or power storage units such as batteries and/or fuel cells configured to generate direct current electric power (DC).
These electrical rotating machines and power storage units are the electric power sources that form the single- or multi-phase power supply network.
The power distribution network, which is connected to the electrical loads, is connected to the power supply network (and therefore to the power sources) via an electric power converter.
The power distribution network is a bipolar network delivering a high voltage direct current (HVDC).
Since the electric power sources are configured to generate a direct and/or alternating current, the electric power converter is a converter configured to operate either as a rectifier, that is, to convert AC electric power into DC electric power (HVAC/HVDC converter), or as a chopper, that is, to transform one DC electric power into another DC electric power (DC/HVDC converter).
It is also known that, in aircraft, the general electric network is provided with an electrical ground which is provided by the aircraft fuselage for metal-fuselage aircraft or by a mesh of electrical conductors, for example of the ESN or CRN type (“Electrical Structural Network” and “Current Return Network”), for aircraft with a fuselage made of a composite material.
Aircraft are known in which the neutral conductor of an electrical rotating machine (generator) is connected to the electrical ground by a neutral electric cable and the metal frames of the on-board systems (loads) are also connected to the electrical ground by ground electric cables.
Thus, the power supply and distribution networks are directly connected to the electrical ground.
When an electrical fault occurs in the general electric network, for example between one phase of the distribution network and the ground, a fault current having a large amplitude, which is greater than a rated current of the distribution network flows within a fault loop.
This fault loop comprises the faulty phase (from the converter to the load), the frame of the on-board systems corresponding to the electric load connected through this faulty phase, the electrical ground cable which connects these systems to the electrical ground of the aircraft, the electrical ground of the aircraft, the neutral electric cable which connects this ground to the neutral conductor of the electrical rotating machine and the phases of this machine (from the machine to the converter).
The protection of the general electric network of the aircraft, and therefore the safety of goods and persons, is ensured by electrical protection equipment configured to detect current surges and/or current leaks and for isolating the system or a portion of the faulty general electric network as close to the fault as possible.
Such electrical protection equipment is generally a circuit-breaker (CB), which may or may not be remotely controlled, or which may even be of the Solid-State Power Controller type (SSPC), or fuses as well as ground fault interrupters (GFI).
It should be noted that such electric power sources each have a substantial weight. It should further be noted that the neutral electric cable, which connects the electrical ground of the aircraft to the neutral conductor of each electrical rotating machine also has a substantial, non-negligible weight.
For obvious reasons of weight reduction, the possibility of isolating electric power sources in such a way that they would not be connected (and therefore not be referenced) to the electrical ground of the aircraft, is envisaged. For example, the neutral electric cable, which connects the neutral conductor of an electrical rotating machine and this ground, would be removed.
In this case, a stray capacitance would be generated between the neutral conductor of this electrical rotating machine and the electrical ground of the aircraft.
It should be noted that a stray capacitance between one or several phases of the power distribution network may also be created when a galvanically isolated electric power converter is used, in particular for coupling a DC electric power source of the power supply network to the power distribution network.
When an electrical fault between one phase of the power distribution network and the ground occurs, a small-amplitude fault current flows within a fault loop, which comprises only the stray capacitance(s). The amplitude of the fault current is small because the stray capacitance(s) has/have a high impedance.
Therefore, such a fault current will be difficult to detect using the electrical protection equipment employed in the case of electric power sources connected to the electrical ground of the aircraft, or in other words, of non-isolated sources, such as those described above. Indeed, the small amplitude of the fault current does not allow such electrical protection equipment to be tripped.
The protection of the general electric network of the aircraft, and therefore the safety of goods and persons, would thus not be ensured.